1. Field of the Invention
This invention relates to magnetic recording elements and processes for preparing such elements. In one of its aspects, this invention relates to magnetic recording elements having magnetic recording layers that are substantially transparent to visible light while exhibiting excellent magnetic recording and reproducing characteristics, and also to processes for preparing such elements.
2. Description Relative to the Prior Art
Conventional magnetic elements that are used for recording sounds or images are generally opaque to visible light regardless of the nature of the magnetic particles used in such elements. However, it is known that magnetic layers formed from magnetic iron oxide dispersed in a binder will transmit infrared radiation. For example, U.S. Pat. No. 2,950,971, issued Dec. 17, 1957, describes an element having multiple sound tracks and comprising a transparent support coated successively with (1) an optical sound track capable of modulating infrared and (2) a magnetic sound track that contains magnetic iron oxide dispersed in a binder and uniformly transmits infrared. The infrared radiation is transmitted through the magnetic sound track to reproduce a previously exposed and processed optical sound track. The element described in this patent may be a motion picture film and the magnetic sound track does not cover that portion of the film used in the projection of images.
As shown in French Pat. No. 1,227,788, granted Mar. 7, 1960 and Canadian Pat. No. 686,172, issued May 12, 1964, a magnetic recording layer may be transparent to visible light when it is very thin and contains a very low concentration of magnetizable particles such as gamma ferric oxide. According to these patents such a layer is coated over a layer containing descriptive material which allows a user to simultaneously hear and see certain subject matter. However, as pointed out in these patents, the electromagnetic characteristics i.e. the magnetic recording and reproducing characteristics of such a layer are inferior to those of conventional magnetic layers as a result of the very low concentration of magnetizable particles.
U.S. Pat. No. 3,782,947, issued Jan. 1, 1974, discloses a photographic product which carries magnetizable particles that are uniformly distributed across the image area of the product. The particle distribution and size(s) are so designed that the composite granularities of the photographic and magnetic recording media are such that the magnetic distribution is essentially transparent in a photographic sense. According to this patent, the photographic image can be viewed via the magnetic distribution, and the magnetic distribution can be employed for recording and playback information.
It is evident that an element containing a magnetic layer which combines transparency to visible light with magnetic recording and reproducing characteristics which are comparable to conventional opaque magnetic layers would represent an advance in the prior art. Likewise, it is evident that it would be desirable to obtain such transparency without the need for matching the granularity of a magnetic medium to that of a photographic medium, as in U.S. Pat. No. 3,782,948. The process for preparing such elements would, of course, clearly provide an advance in the prior art.